A circuit for generating data of a letter to be displayed on a screen is widely used to display a number, a symbol, an alphabetical letter, etc. (simply called a "letter" hereinafter) which are superimposed on a video image displayed on a CRT screen of a raster scan type. A typical example of such a letter is a channel number, etc. displayed on a video image which has been displayed on a CRT screen of a television set.
A conventional circuit for generating data of a letter comprises a character memory for storing data of letters to be displayed, and a video memory for temporarily storing an address of the character memory to be accessed. In this circuit, predetermined address information is written into the video memory to access the character memory, so that data of a letter to be displayed is read from the character memory by the address information. The data of the letter read from the character memory is supplied to a display circuit including a CRT screen, so that at least one letter is displayed at a predetermined position on the CRT screen by superimposing the data of the letter on data of a video image at a predetermined time or by replacing a portion of the video image data with the letter data.
In such a conventional circuit for generating data of a letter, it is required in recent years that a letter selected from a plurality of letters is displayed in a predetermined color which is different from that of the remaining letters. It is also required that a background of the displayed letter selected from plural backgrounds is displayed in a predetermined color which is different from that of the displayed letter, and from those of the remaining backgrounds. For this purpose, it is necessary that a memory for storing data designating a color of a letter to be displayed and data designating a color of a background corresponding to the letter is provided.
However, where such a memory is provided to realize the purpose described above, a disadvantage occurs in that the storage capacity of a memory is increased to result in the increase in chip area of an integrated circuit necessary for generating data of a letter. As an occupied area of the memory is increased, space considerations in for instance, a television set, prevent other functions from being added thereto. This disadvantage becomes considerable, as the number of letters to be stored is increased.